Going to weld the joints gradually and make a weld plan.
One of the key things to consider as you plan your welding sequence is to balance stresses. Generally the work pulls toward the weld as it cools. A Butt weld will warp toward the side the bead is on. If you can, set up the weld sequence so you balance those stresses by immediately making another weld that tries to pull things back in the opposite direction you can largely eliminate welding distortion. Then the two welds will oppose each other and lead to minimum movement.
Think about intentionally deforming the item slightly in the opposite direction in the initial fit up, so that the weld pulls it back into the neutral position.
In the case of low alloy steels, don't be afraid of heating areas to allow the stresses to dissipate. After a weld is made that distorts the piece, jack it or pull it as necessary to apply a stress in the opposite direction then heat the weld area to a dull red and let it stress relieve back into the intended position.
As mentioned fit up and welding is as much an art as a science. My Aunts father in law worked for 20+ years at a steel fabrication company here in Denver making bridge girders and other heavy structural steel assemblies, and told endless tales of how they anticipated the movement of the steel as they fabricated it. Girders were built with a camber in mid span so when they were in place their weight would pull them down to be straight rather than sagging.
He would fit up pieces that had lots of weld on one side so the welding shrinkage would pull it back into true and they were not afraid of getting out large acetylene torches and heating areas to a dull red after welding to let the stresses relax as the slightly plastic hot steel yielded to the welding stresses. Sometimes putting large chain fall come-a-longs on the pieces and pulling them hard in the direction opposite of the direction the welding shrinkage would act as the welds cooled.
Lots of carefully applied brute force is used in heavy fabrication to make things actually be the shape and size intended when the fabrication was done.
Larry